A formal fallacy is a flaw in the structure of an argument — an error you can identify from its logical form alone, without knowing anything about the subject matter. When the form is invalid, the conclusion doesn't follow even if every premise happens to be true.
That's what separates formal fallacies from informal ones, where the problem lies in the content, context, or use of language rather than the shape of the reasoning. Every formal fallacy is a kind of non sequitur — Latin for "it does not follow" — because the inferential step between premises and conclusion is broken.
Because the defect is structural, formal fallacies turn up most often in deductive reasoning, where a conclusion is supposed to follow with certainty. The 8 below cover the classic structural errors.
Affirming the consequent — the most common formal slip.
If it rained, the street is wet.
The street is wet.
Therefore, it rained.
Invalid. The street could be wet for many reasons — a burst pipe, a passing street-cleaner. The form "If P then Q; Q; therefore P" doesn't hold: confirming the result tells you nothing about which cause produced it.
Every formal fallacy in the library, each an error in the argument's form.
Affirming the ConsequentAssuming that because the result is true, one specific cause must be true.
Bad Reason FallacyJudging a conclusion false simply because it rests on a bad reason.
Denying the AntecedentAssuming that if the cause is false, the result must be false too.
Non SequiturA conclusion that simply doesn't follow from the premises given.
Propositional FallaciesErrors in how 'and', 'or', and 'if' connect statements logically.
Quantification FallaciesMistakes from misusing quantifiers like 'all', 'some', or 'none'.
Sunk Cost FallacyContinuing something only because of what you've already put in.
Syllogistic FallaciesFaulty deduction that draws a flawed conclusion from two premises.
